1. Field of the Invention
The present invention relates to instruments having scanning probes, and in particular, to scanning probe microscopes (SPM's) with piezoelectric scanners and the capability to vary the size of the scanned area within the range of the scanner.
2. Discussion of the Background
Instruments that contain scanning probes and require precise control of the probes are becoming increasingly important in research and industry. SPM's are one type of these instruments which provide high resolution information about the properties of surfaces. One common use of SPM's is imaging, and some types of SPM's have the capability of imaging individual atoms. Along with imaging, SPM's can measure a variety of surface properties, with detail over the range from a few angstroms to hundreds of microns. For many applications, SPM's can provide lateral and vertical resolution that is not obtainable from any other type of device.
SPM's operate by scanning a probe, usually some form of very sharp tip, over a surface in a raster scan. The tip interacts with the surface, and this interaction is measured. A variety of interactions can be used, but typically these interactions are confined to a very small area around the tip. For example, the first SPM developed was the scanning tunneling microscope or STM. Such a device is described in U.S. Pat. No. 4,343,993, to Binnio et al. The STM places a sharp, conducting tip near a surface. The surface is biased at a potential relative to the tip. When the tip is brought near the surface, a current will flow in the tip due to the tunneling effect. Tunneling will occur between the atom(s) closest to the surface in the tip and the atoms on the surface. This current is a function of the distance between the tip and the surface, and typically the tip must be within 20 angstroms of the surface for measurable current to be present. STM's require an electrically conducting surface, or an insulated surface coated with a conducting layer.
Another SPM, the atomic force microscope (AFM), similarly scans a tip across a surface. The tip in this case is mounted on a free end of a lever or cantilever, which is fixed at the other end. The tip is brought in contact with a surface, such that the force of interaction of the tip with the surface causes the cantilever to deflect. If the tip exerts a sufficiently small force on the sample, then the interaction can be confined to a very small area. Other interactions between a probe and a surface have been used successfully, such as electric and magnetic field detection, near-field optical effects, and ionic conductance.
SPM's require the capability to measure the interaction between the probe and the tip. In the case of the STM, the currents are easily measurable, on the order of nanoamps. For the AFM, very small deflections must be measured which has been accomplished both with optical techniques, such as beam deflection or interference, and tunneling methods.
SPM's also require a mechanism to scan the tip over the surface in a raster pattern. Ideally, this mechanism should have controllable motions both laterally and vertically to the surface that are of a size to take advantage of the resolution provided by the tip-surface interaction. This mechanism must also be very rigid, as the interactions between tip and surface require extremely high stability. Most SPM's currently use piezoelectric actuators that are constructed to achieve three-axis scanning with very high resolution. The most common type of scanner used for SPM's are piezoelectric materials formed into the shape of a tube. Electrodes are placed on this tube such that longitudinal bending motions constitute the lateral scanning motions, and shortening or lengthening of the tube constitute the vertical scanning motions. These tubes are very rigid. Such scanners have been made with resolution of fractions of an angstrom. Scanners of this type have also been constructed with maximum scan sizes of over 100 microns.
SPM's typically do not operate by using the tip-surface interaction directly to form an image of the surface, but rather control the interaction very closely while scanning the surface. For example, in an STM, the tip-surface separation cannot be allowed to vary because the tunneling current decreases rapidly if the separation increases. In an AFM, the force and therefore the cantilever deflection must be kept low. Therefore SPM's are usually operated in a feedback mode, where, as the sample is scanned laterally in a raster pattern, the sample or tip is adjusted vertically to maintain the measured interaction parameter at a constant, predetermined value. The adjustment information as a function of lateral position forms a topographic map of the scanned surface. The feedback implementation is critical to the operation of an SPM, as high performance feedback systems allow for faster, more accurate imaging.
The ability of piezoelectric materials to produce very small, controlled incremental movements is critical to the operation of SPM's, as no other positioning devices exist with adequate resolution, response time, or controllability. However, piezoelectric materials, although making SPM's possible, do not have ideal characteristics. The actual characteristics of these materials have a significant effect on performance.
Piezoelectric scanners exhibit a motion that is a non-linear function of applied voltage. The inventors have addressed this issue in U.S. Pat. No. 5,051,646 and co-pending application Ser. No. 07/447,851 filed Dec. 8, 1989 whose disclosures are herein incorporated by reference.
Much prior art also exists on the subject of piezoelectric non-linearity. One effect that has not been addressed in the prior art is the sensitivity of the piezoelectric scanner as a function of the applied voltage. Sensitivity in this context is the motion response of the scanner to the applied voltage, usually expressed for SPM scanners in angstroms/volt. This sensitivity number is not constant over the range of applied voltage.
The lateral scanning motions in an SPM create a raster pattern, as illustrated in FIG. 1. FIG. 1 illustratively represents raster scan lines for one scan. A typical scan is comprised of hundreds of scan lines, a scan line being one excursion back and forth in the x-direction. The scanner moves back and forth in one lateral direction, x, at a relatively high rate while the scanner is displaced back and forth in an orthogonal lateral direction, y, at a much lower rate. Thus the scanner repetitively scans a rectangular area. This motion is achieved by applying waveforms shown in FIG. 2 to the corresponding x and y electrodes 32 and 34, respectively on the scanner 30 as illustrated in FIG. 3. Typically, the frequency of the x-direction waveform is hundreds of times higher than that of the y-direction waveform.
Historically, triangular waveforms are applied to achieve the desired motions, similar in function to the triangular waveforms used to create the raster scans in cathode ray terminals or television sets. The inventors have found, however as described in copending application Ser. No. 07/447,851, that non-linear waveforms that compensate for the piezoelectric non linear behavior are preferable, but most SPM's use triangular waveforms.
The waveforms used to drive the scanners may span the entire range of voltage available, and thus create the maximum scan size, or the waveforms may be scaled to be a portion of the total range. In this way the scan size may be changed. In practice, the size may be varied from the maximum down to the smallest size compatible with the electronics used to create the drive waveforms. Prior art SPM's simply scale the drive waveforms to change the scan size.
However as noted above, the sensitivity of the scanner is a nonlinear function of the applied voltage. For a typical scanner used by the inventors, the scan size S may be represented by: EQU S=aV+bV.sup.2 =V(a+bV) (1)
where V is the maximum voltage applied, a is the linear sensitivity factor, typically determined experimentally, and b is termed the derating factor and is positive. Higher order terms could also be included for a more accurate model of scanner behavior. Typical values are a=2200.ANG./V and b=3.ANG./V.sup.2 for a scanner of this type with a maximum scan size of 100 microns.
Thus, when the drive waveforms are scaled, the sensitivity increases as the scan voltage increases. Moreover, the inventors have found that the sensitivity change has a cycle lag, in that when the scan size is changed, the sensitivity does not change immediately to the new value. The inventors have discovered that the time period over which the sensitivity settles is related to the scanning motion, and thus the sensitivity changes at a different rate in the x direction than in the y direction. After a change in scan size, the sensitivity remains at the value for the previous scan size for a few cycles of the scanner. The sensitivity in the x direction settles over a few cycles, while the sensitivity in the y direction also settles in a few cycles but much more slowly because the y motion cycles at a much lower rate.
After several scanning cycles, the sensitivity of the scanner will settle to a steady value. The inventors have found by experience that if the scan size is suddenly changed, the scanner cannot "know" what the new end point of the scan motion will be until that point is actually reached. Thus, the settling process cannot begin until a scan cycle is complete.
This effect causes the substantial distortion and inaccuracies found in existing SPM's when scan sizes are changed. Prior art SPM'S rescale the drive voltages and immediately begin scanning at the new scan size, as shown in FIG. 4A. The prior art SPM operates at a first scan size (step 40) illustrated by the scanning motion shown in FIG. 4B after the scanning sensitivity has settled to a steady value. The scan size changes (step 41) and the SPM immediately begins a new scan at the changed scan size (step 42), providing new scan motions shown in FIGS. 4C-4E. For convenience, the motions are shown where the scan size was changed at the beginning of a new scan. The x sensitivity settles quickly to its new value during the scan, giving a scan motion as shown in FIG. 4C. The y sensitivity settles out over one or more frames, such that during the first frame the x and y sensitivities are very different, giving in general a rectangular scan not a square one (FIG. 4D). This effect then goes away as the scanner is scanned through the frames, as shown in scanning motions 4C-4E, which are in that time order but not necessarily successive frames. In FIG. 4E the sensitivities are settled and the scanner behaves as expected.
In general, for prior art SPM's, the first scan after a change in scan size is too distorted to be useful, and data is not acquired until the second or the third scan, i.e., after both x- and y-direction sensitivities have settled. For large scans of tens to hundreds of microns, the time for the first scan can be as long as several minutes, which can be very inconvenient for all applications and limits the usefulness of SPM's for production-oriented tasks where the speed of accurate data acquisition is very important.